S3C6400/SMDK6400: fix stack_setup in start.S

Fix stack_setup to place the stack on the correct address in DRAM
accroding to U-Boot standard and remove conditional compilation by
CONFIG_MEMORY_UPPER_CODE macro that is not necessry. This macro
was introduced and used only by this board for some unclear reason.

The definition of this macro is also removed because it's not
referenced elsewhere.

Signed-off-by: Seunghyeon Rhee <seunghyeon@lpmtec.com>
Tested-by: Minkyu Kang <mk7.kang@samsung.com>